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Sutron Corporation, 22400 Davis Drive, Sterling, Virginia 20164-4444
20
aXEou!
where o is adjustment value with units u. u can have units
0=mB (hPa), 1="Hg, 2=kPa, 3=mmHg, 4=Atm, 5=psia,
or 9=user units.
For example, the command:
aXE+20.2+0!
would set the elevation offset pressure to 20.2 with units of mB (hPa).
The other command used to set the elevation offset is the XS command. This command causes the sensor
to make pressure readings and automatically compute a new elevation offset. You can use this command
only if you have a stable, known pressure on the sensor. The command has the format:
aXSdu!
use this form when the sensor is at a stable, known
pressure. The d represents the desired reading and u the
units.
If the sensor was stable at 1005.23 mB, the following command would adjust the elevation offset to insure
the 1005.23 mB reading:
0XS+1005.23+0!
If the sensor was under pressure and stable at 29.921 inches of mercury, the following command would
adjust the elevation to ensure the 29.921" Hg reading:
0XS+29.921+1!
When the ACCUBAR
®
is done with the self-calibration, the new elevation offset is stored into memory. A
subsequent aD0! command will display this offset in units of mB. The offset can also be displayed using
the M3 command. The returned value will be in the current units of pressure.
Analog Output Range
The -2 version supports analog output. The output range is 0 to 5 volts. As shipped from the factory, this
corresponds to 600 to 1100 mB. The analog output is driven by a 12 bit D/A converter. This means that
the output changes in discrete steps of about 1.25 mV. Analog transmission of data is less accurate than
digital transmission. There are three contributors to this error: Error in the transmitted value; noise and
voltage drops picked up during transmission through the cable; conversion errors at the receiving end. For
the ACCUBAR
®
, the error in the transmitted value is going to be the error in the digital value plus a voltage
error of the output. For the receiving end (data recorder, logger, panel display), there is a quantization error
plus an accuracy error when the analog voltage is converted to a digital value. The best resolution of a 12
bit A/D on a 0 to 5 scale is 1.25 mV. If the scale is wider or the number of bits is less, then the resolution is
even coarser.
Most users will want to customize the output range to maximize the accuracy of their equipment over the
range of interest. With 600 to 1100 mB corresponding to 0 to 5V, a 1 mV error corresponds to a 0.1 mB
